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Height references

Observing the sea level and using these measurements to determine several height references is one of the basic hydrographic applications of tide gauging to reduce soundings to the same vertical datum, the chart datum.

To measure the water height, a reference with two fundamental qualities must be established.

It must be clearly recognized, which means that when a height is given, we know where it was measured from;

It must be permanent, which means that it must be defined precisely or be positioned relative to permanent benchmarks.

Problems in selecting and locating the vertical datum

The choice and location of vertical datums are fundamental problems in hydrography.

These problems are long-standing, but new technologies such as satellite observations and new computer technologies are profoundly changing the way they are addressed.

What with positioning systems such as GPS / GLONASS / Galileo, altimetry, the unique international geodetic reference ITRF, the improved determination of the Earth's gravitational field and ever more powerful hydrodynamic models, new systems of reference are being considered.

However, traditional systems are still in use, and it is necessary to precisely relate these reference frames to one another.

Some typical sea levels in France

In France, some typical sea levels are used as a reference for height measurements, including:

Mean sea level at Marseille

"The mean sea level at Marseille" at the end of the nineteenth century defined the reference for a zero officialised on January 13, 1860 called "zero of Bourdaloue" or the zero of France's fundamental levelling benchmark.

It was transposed by geometric levelling (spirit level) for the general levelling of France. From 1857 to 1970, three levelling networks succeeded one another: Bourdaloue networks, Lallemand networks and IGN 69 networks.

The altitude of the fundamental benchmark was set based on the average tidal observations made in Marseille between 1885 and 1897. The fundamental benchmark is a platinum rivet sealed in the Anse Calvo tide gauge building.

Lowest astronomical tide

The lowest astronomical tide (since Beautemps-Baupre in 1838, "father" of modern hydrography) is in principle the level of the French Navy cartography. But some difficulties have arisen due to inaccuracies of old determinations, and the secular sea level trends.

Hydrographic zero or Chart datum

The chart datum is defined in France as "the level of the lowest astronomical tide."

In a given location, in areas under the responsibility of France, hydrographers have always tried to choose the chart datum so that the water depth available for navigators is always at least equal to the depth indicated on the charts.

Used starting in 1838 by Beautemps-Baupre, coordinated by the Hydrographic Service of the Navy and published in the Pilote Français, the observations were used to establish in our ports the zeros to which the soundings were reduced.

These zeros, defined previously, are still being used on French charts except for some areas:

near Brest and Saint-Nazaire;

Gulf of Morbihan

where respectively on 1 January 1996 and 1 January 2003, corrections were made to correct significant differences between the level of the lowest astronomical tide and the historical chart datum.

Definition of chart datum in other countries

Different definitions of chart datum are used by foreign hydrographic services, including:

Mean Lower Low Water (United States)

Mean Low Water Springs (Germany)

Mean Lower Low Water Springs (Netherlands)

Mean Lower Low Water Springs occurring every six months (Indonesia)

Indian Spring Low Water (India, Japan ...): mean level minus the sum of the amplitudes of the four main tide components.

Since 1996, the International Hydrographic Organization (IHO) recommends the use of lowest astronomical tide (LAT), that is to say, the standard used in France for over a century and a half.